Specialized mounting systems have been developed for various machinery arrangements in different technical fields. It has been found that for relatively large, bulky machines such as gas turbines, large electric generators and turbo-compressors, mounting systems must often be developed, which can provide robust support and stability to the machinery components, while being tailored to specific machine designs. Mounting strategies often must further account for the environmental conditions, in which a particular machine will operate.
The oil and gas industry provides a number of exemplary situations, where particular operating conditions of very large rotating machines require special mounting systems. Rotating machine arrangements typically include a prime mover, such as a gas turbine or electric motor, driving a load comprised of a rotating machine, e.g. an electric generator or a turbo-compressor. In the context of the present description and annexed claims, the term turbo-compressor is used to designate a dynamic-type compressor, such as an axial or centrifugal compressor.
The rotating machines are often arranged on a base plate or base frame, forming a single module arrangement. The base frame is in turn mounted on a supporting structure, such as an off-shore platform, or the deck of a marine vessel, or any other steel structure in general.
Typical applications of large rotating machines in the oil and gas industry include natural gas liquefaction facilities. Natural gas extracted from an offshore gas field is chilled and liquefied for transportation purposes. Refrigerants are processed in a chilling process for cooling and liquefying the natural gas. Turbo-compressors driven by gas turbine engines are used for processing the refrigerant in the refrigeration cycle. Gas turbine engines are also used for electric energy production purposes, for driving an electric generator. Large rotating turbo-compressors are also used in the field of oil and gas for gas injection and gas lift applications.
Base plates for rotating machines of this kind must be designed to resist high static and dynamic loads, due to the load of the rotating machines, as well as to the operation thereof. Dynamic loads include operative loads related to normal operation of the machine, as well as accidental and environmental loads. The former are due to abnormal operating conditions of the rotating machines, e.g. due to unbalances caused by blade losses in the turbine or to extreme events such as explosions.
The latter can be due e.g. to wave or wind action on the vessel or off-shore platform or seismic in case of fixed platform, where the rotating machines are installed.
An otherwise flat, generally planar vessel deck may experience torsional motion under the influence of wave action or other vibration and mechanical stresses, and in turn may transmit the torsional motion to the base plate, whereon the rotating machines are mounted.
While in on-shore applications the rotating machines are usually mounted by means of a multi-point, hyperstatic system (also named statically undeterminable or statically indeterminate systems) on the ground, hyperstatic mounting is generally considered unsuitable in off-shore applications, due to the above mentioned motions due e.g. to wave action or the like.
Twisting of a vessel deck due to wave action, for instance, can cause the mounting points of a hyperstatic, multi-point system to actually move out of the originally intended mounting plane. This in turn causes misalignment of the rotation shafts of the train of rotating machines mounted on the base frame or base plate. In case of equipment having low tolerances for misalignment of components, the above situation can be fatal.
In an attempt to address the above problems, three-point mounting systems have been developed. A three-point mounting system includes a base plate or base frame having an upper surface, where the rotating machines are installed, and a lower surface, where three supporting members are arranged. The supporting members connect the base frame to the deck of a vessel, or off-shore platform, or on any other supporting structure. The supporting members are located at the vertices of a triangle, which can be centered with the centerline of the base plate, or with the shaftline of the rotating machines arranged on top of the base plate or with Center of Gravity axial line.
The design of the supporting members is such as to provide an isostatic connection between the base plate and the supporting structure. For this purpose, each supporting member provides constraints such as to allow all rotating movements. Two supporting members are sliding in one direction while one supporting member is fixed also in translating movements, The single degree of freedom left by each of the two sliding support members allow e.g. thermal growth of the base frame with respect to the deck or other supporting structure, due to the heat generated by the turbomachinery during operation. This isostatic connection accommodates any displacement between base frame and supporting structure, without inducing additional deflection in the base frame that would negatively affect alignment of the rotating machines. Moreover, use of a three-point isostatic connection simplifies the design of the supporting structure, as it does not modify the global stiffness thereof.
Typically, gimbals, i.e. spherical joints mounted on pivoting pins, or anti-vibration mounts can be used as supporting members in this kind of three-point, isostatic mounting arrangements.
Three-point, isostatic connection systems have, however, some drawbacks. In particular, since the entire static and dynamic load must be supported by three supporting members only, these latter have often large dimensions. Moreover, dynamic and static loads on the deck of the vessel, or offshore platform, where the turbomachinery train is installed, are concentrated in three points.
Load concentration requires the supporting members and the deck to be dimensioned to withstand normal operating loads, as well as emergency or accidental loads like, for example, blasts load due to hydrocarbon explosions.
These aspects become particularly critical in case of very large machine components. The need for using three-point mounting systems, in order to avoid the disadvantages of multi-point, hyperstatic systems, limits the dimension of the rotating machines, which can be used.
Also the package surrounding the rotating machines mounted on a base plate can be supported by the base plate and contribute to the overall weight of the system. Thus, the use of three-point mounting systems can be difficult in case of heavy packages supported by the base plate, or can limit the maximum dimension and weight of the package.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.